A coating type magnetic recording medium is prepared by dispersing ferromagnetic particles in a binder and coating the thus obtained coating solution on a nonmagnetic support and, if necessary, conducting magnetic orientation and smoothing treatment thereon.
Recently, a magnetic recording medium (hereinafter referred to as "a magnetic tape") has been required to have improved characteristics and various improvements have been done. One of these characteristics is S/N ratio. It has been suggested that one approach to improve S/N ratio of a coating type magnetic tape is that ferromagnetic particles having a lower particle size and having a higher specific surface area be used. In this instance, it is necessary that ferromagnetic particles should be homogeneously dispersed in a binder and that the surface of the coated magnetic layer should be smoothed. However, as the particle size of the ferromagnetic particles becomes lower, it becomes more difficult to disperse the ferromagnetic particles homogeneously in the binder, thereby deteriorating the orientation of ferromagnetic particles and smoothness of the surface of the magnetic layer. As a result, a higher S/N ratio can not be obtained.
In order to improve dispersibility of ferromagnetic fine particles, various binder dispersing assistants and various mixing and kneading methods using kneading machines, such as a two-roll mill, a three-roll mill, an open kneader, a kneader under pressure, or a continuous kneader, have generally been considered.
The above described two-roll mill, three-roll mill, open kneader, kneader under pressure, and continuous kneader are used to mix and knead a coating composition for a magnetic tape. The mixing and kneading degree varies depending upon characteristics of ferromagnetic particles, additive amounts of a binder and a solvent with using anyone of the above described kneaders. Therefore, it has been extremely difficult to obtain a coating solution having a desired degree of dispersion. It has been found that as the particle size of magnetic particles becomes lower, it becomes more difficult to obtain a desired degree of dispersibility. Accordingly, it has been difficult to obtain a magnetic tape having a good dispersibility and having a high S/N ratio with a conventional dispersing method when the particle size of the magnetic particles is very small.
The present inventors found that when the particle size of ferromagnetic particles is low, that is, when the specific surface area thereof is high, it is difficult to disperse the magnetic particles, but it becomes easy to disperse the magnetic particles when the content of adsorbed moisture is increased. According to the above discovery, the present inventors provided a method for preparing a magnetic recording medium which comprises adjusting the adsorbed moisture content of ferromagnetic particles having a specific surface area S.sub.BET of 35 m.sup.2 /g or more to 0.8 wt % or more, pre-dispersing the ferromagnetic particles in a solvent with a binder, adding an additional binder and a lubricating agent thereto, mixing and dispersing the mixture, and coating the thus prepared coating solution on a support, as disclosed in U.S. Pat. No. 4,569,856 corresponding to Japanese Patent Application (OPI) No. 187931/85 (the term "OPI" as used herein means an "unexamined published patent application").
In accordance with the above method, the problems of the prior art have been, in a way, solved, but there are still problems in that it requires a long term to mix and knead upon preparation of a magnetic coating composition and the dispersibility is still rather insufficient.